We begin with the crises in egypt. All day long the world has watched as this life or death drama unfolded, a nation so key to the united states hanging in the balance. You're looking live at tahrir square. It is filled with celebrations tonight now that theresident has been ousted but there is also fear that there will be bloodshed. The president's supporters will not give in easily. They are vowing revenge. The images of this day tells the story. The people of egypt demanding their president step aside. The military stepping in, weapons drawn. And here is the very latest. The egyptian military is effectively in control. The u.S. State department is now urging all americans to leave egypt. We will explore what all this means to the u.S. Which relies so heavily on egypt to bring stability to such a dangerous region. Our team is spanned out across the globe but we begin with alex marquardt in the center of it all in cairo. Reporter: This is the place where two and a half years ago they deposed a dictator. Today they're doing the same to the first democratically elected president and tonight he's fighting back. As the seconds tick down toward the ultimatum deadline, the crowd on tahrir square swelled to tens of thousands. As the hours passed with no word, confusion mounted. You have no idea who's running the country now? Right now we have no idea. Reporter: Many already believed it was over for morsi. Fireworks exploded. Army military helicopters flew overhead to roars below, a sign the people felt that the army was on their side. It feels like a party tonight. It is. Reporter: It is? Don't you feel it? I feel it. Reporter: Meanwhile the military was deploying in the streets of the capital. Spokesman tweeted there is a full military coup in egypt. Then they announced morsi is out. The head of the military said he was respond k to the call of the people, suspending the counsel's tugs and naming the top constitutional judge to take over until new presidential elections can be held. Tahrir square, the birth place of egypt's revolution two and a half years ago erupted once again. Since sunday millions have poored into the streets across egypt in historically massive protests, demanding that morsi step down and calling for early elections. This is the moment we've been waiting for for three years and a half. We have been waiting for this moment. Reporter: While those in tahrir celebrated, his responders were angered. The once outlawed muz lum brotherhood isn't going down without a fight. President morsi responded with an online video, rejecting the dissolution of the constitution and accusing the army of stealing the revolution. Whoever planned this he said wants to spill the blood of the egyptian people. Alex marquardt. Abc news, cairo. As we said, egypt has long been a critical partner of the united states and martha raddatz has more now on america's view of this particular power struggle. Reporter: The chaotic scenes from egypt today put the us in an impossible position. Watching angry supporters of morsi. On the other side, jup lent supporters. Washington cannot support a military takeover of a democratically elected president. We are not taking sides. Reporter: Yet it is clear that the administration was not happy with mohammed morsi and his failure to embrace the democratic process. He must do more to be truly responsive and representative to the justified concerns expressed by the egyptian people. Reporter: For america, egypt is a critical partner in support of democracy and security in the region. Egypt is an anchor for the region as a whole. Reporter: It's the largest arab country, one in every four arabs live in egypt, it sits on top of the suez canal which is how eye naval forces get in and out of the gulf and the oil gets to the markets. The 1.6 billion dollars the u.S. Gives in aid, most of it military aid is second only to what we give israel. With the region already in crises, with syria's deadly civil war, the nuclear threat from iran and unstable iraq, what happens next in egypt is a grave concern. This coup is opposed and that could lead egypt down the road of civil violence which would be terrible. Joining us now with martha raddatz and christiane amanpour. Martha, despite all that money we give to egypt, we're really on the site lines here. We have little or no influence over president morsi are the military in the opposition. Not over morsi for sure. The military to military relationship between the u.S. And egypt is very strong. They train officers here, pilots. In fact, there's an abe rams paint factory in egypt that is run jointly. The u.S. Seems to believe they will not take advantage of the power volume and the egyptian military has assured the pentagon this is only temporary. As you say, elizabeth, we'll see. You were the last jurn list to view ma bare ek before he was transformed. A democratic elected president. You're absolutely right. It's a paradox. You're going to see united states top officials turning themselves into pretzels trying not to call this a coup. If it looks and acts like a coup, the u.S. Demands that military aid and other aid is halted. There's a lot of wiggle room but some are saying this is an out and out coup, stealing the revolution and the ante morsi people saying this is a vote and a reset of the revolution. This is a country divided. We saw the hundreds of thousands in tahrir square celebrating, hundreds of thousands protesting six miles away. What are the chances violence might break out? That is a worry. There has been some violence overnight last night. Morsi has called for peaceful resistance but he said he's still the legitimate president and he doesn't want people to accept what happened. It's very crucial. It's a test case and vital for the united states. Now that president morsi is out, who is in charge as they say in egypt? Well, they say they have an interim group in charge right now. It is pretty much unclear. They have dissolved the constitution. One particular person they say is in charge but we'll have to see in the coming days whether that takes or not because right now the military is in charge. And only the military. Martha raddatz, christiane amanpour. Thank you so much. Our coverage in egypt tonight.

This transcript has been automatically generated and may not be 100% accurate.

Egypt May be on the Brink of a Civil War

Less than 24 hours remain for President Mohamed Morsi to respond to military imposed deadline.

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